For a contact lens to properly fit a wearer, it is important that a posterior radius of curvature of the lens closely match the frontal curvature of the wearer's cornea. One parameter of fit is the base radius of curvature, or base curve. The actual base curve of a soft contact lens should be within 0.1 millimeter of that prescribed for a particular patient. For "extended wear" contact lenses, i.e., those worn for more than twenty-four hours, the fit should be even more accurate.
A soft contact lens is made from a hard plastic material which is capable of absorbing lacrimal fluid. The hard plastic material is lathed and polished into a hard lens with a certain base curve. When this hard lens material is contacted with a saline solution, it absorbs the solution and becomes soft and pliant. The resultant base curve of the hyrated lens may vary markedly from the base curve in the non-hydrated state. Since the soft contact lens assumes its natural shape only when suspended in a solution, it is only then that the base curve can be measured.
Soft lens manufacturers supply nominal base radius values for their lenses. Experience indicates, however, that the nominal values supplied by the manufacturer cannot be used to classify one lens base radius relative another (i.e. larger or smaller) and most certainly are not accurate indications of absolute base radius.
Past methods to measure the base curve have included electrical and ultrasonic methods which require expensive equipment not suited to an opthamologist's or optometrist's office. The lack of precision in the manufacturers specified base radius values casts doubt on the accuracy of these sophisticated measurements.